The present invention relate generally to encoding of an acoustic source signal such that a corresponding signal reconstructed on basis of the encoded information has a perceived sound quality, which is higher than according to known encoding solutions. More particularly the invention relates to encoding of acoustic source signals to produce encoded information for transmission over a transmission medium.
There are many different applications for speech codecs (codec=coder and decoder). Encoding and decoding schemes are, for instance, used for bit-rate efficient transmission of acoustic source signals in fixed and mobile communications systems and in videoconferencing systems. Speech codecs can also be utilised in secure telephony and for voice storage.
The trend in fixed and mobile telephony as well as in videoconferencing is towards improved quality of the reconstructed acoustic source signal. This trend reflects the customer expectation that these systems provide a sound quality at least as good as that of today""s fixed telephone network. One way to meet this expectation is to broaden the frequency band for the acoustic source signal and thus convey more of the information contained in the source signal to the receiver. It is true that the majority of the energy of a speech signal is spectrally located between 0 kHz and 4 kHz (i.e. the typical bandwidth of a state-of-the-art codec). However, a substantial amount of the energy is also distributed in the frequency band 4 kHz to 8 kHz. The frequency components in this band represent information that is perceived by a human listener as xe2x80x9cclearnessxe2x80x9d and a feeling of the speaker xe2x80x9cbeing closexe2x80x9d to the listener.
The frequency resolution of the human hearing decreases with increasing frequencies. The frequency components between 4 kHz and 8 kHz therefore require comparatively few bits to model with a sufficient accuracy.
One approach to the problem of encoding an acoustic source signal such that it can be reconstructed by a receiver with a relatively good perceived sound quality is to include, for instance, a post filter operating in serial or in parallel with the regular encoding means, which generates an encoded signal in addition to the primary encoded information. Coding solutions involving post filtering exist for narrowband acoustic source signals (typically having a bandwidth of 0-3.5 kHz or 0-4 kHz). However, if these narrowband solutions are used for transmitting acoustic source signals with larger bandwidths, the signals are reconstructed with a comparatively poor sound quality. The reason for this is that both the basic coder solution and the enhancement solution are optimised for preserving the characteristics of narrowband signals. In fact, the enhancement coding can, under unfortunate circumstances, even worsen the situation with respect to perceived sound quality.
Moreover, the known speech codecs operating at rates below 16 kbps, typically in mobile applications, in general show a relatively low performance for non-speech sounds, such as music.
Thus, none of today""s codecs or coding schemes provide a solution through which a broadband acoustic source signal can be encoded and reconstructed with a satisfying perceived quality. Furthermore, perceptually improved narrowband coding solutions are demanded for certain applications.
The object of the present invention is therefore to alleviate the above problems and make possible an efficient encoding, transmission and reconstruction of broadband and narrowband acoustic source signals having a substantially improved perceived quality in comparison to the known solutions.
According to one aspect of the invention the object is achieved by a method of encoding an acoustic source signal as initially described, which is characterised by an enhancement spectrum comprising a larger number of spectral coefficients than the number of sample values in a target signal frame respective a primary coded signal frame. The increased number of spectral coefficients in the enhancement spectrum in relation to the number of sample values in the other signals thus provides a basis for accomplishing the desired improvement of the perceived sound quality.
According to a further aspect of the invention the object is achieved by a computer program directly loadable into the internal memory of a computer, comprising software for controlling the method described in the above paragraph when said program is run on the computer.
According to another aspect of the invention the object is achieved by a computer readable medium, having a program recorded thereon, where the program is to make the computer control the method described in the penultimate paragraph above.
According to yet another aspect of the invention the object is achieved by a method of decoding encoded information having been transmitted over a transmission medium as initially described, which is characterised by producing an enhanced coded signal by extending a relevant reconstructed primary coded signal frame to comprise as many sample values as there are spectral coefficients in the enhancement spectrum.
According to still a further aspect of the invention the object is achieved by a computer program directly loadable into the internal memory of a computer, comprising software for controlling the method described in the above paragraph when said program is run on the computer.
According to an additional aspect of the invention the object is achieved by a computer readable medium, having a program recorded thereon, where the program is to make the computer control the method described in the penultimate paragraph above.
According to another aspect of the invention the object is achieved by a transmitter for encoding an acoustic source signal to produce encoded information for transmission over a transmission medium as initially described, which is characterised in that an enhancement spectrum comprises a larger number of spectral coefficients than there are sample values in an incoming target signal frame respective an incoming primary coded signal frame. An enhancement estimation unit in the transmitter extends a relevant target signal frame and a relevant primary coded signal frame such that they each comprise as many sample values as there are spectral coefficients in the enhancement spectrum.
According to yet another aspect of the invention the object is achieved by a receiver for receiving and decoding encoded information from a transmission medium as initially described, which is characterised in that an enhancement unit extends an incoming reconstructed primary coded signal frame to comprise as many sample values as there are spectral coefficients in the enhancement spectrum.
According to still another aspect of the invention the object is achieved by a communication system for the exchange of encoded acoustic source signals between a first and a second node comprising the proposed transmitter, the proposed receiver and a transmission medium for transporting encoded information from the transmitter to the receiver.
The proposed extended number spectral coefficients in the enhancement spectrum, of course, increases the frequency resolution for the corresponding signal. This provides a basis for many beneficial effects, particularly with respect to perceived sound quality. An improved frequency resolution namely means that more of the perceptually important information contained in the source signal can thus be encoded and forwarded to the receiver.
Furthermore, it is preferable from a computational point of view to utilise signal frames, which include a number of sample values that is suitable for fast Fourier transformation (FFT), for instance, powers of the integer two. The proposed solution provides a perfect freedom to chose an ideal frame size with respect to this.
The invention thus both accommodates an improved perceptual quality and a computationally efficient solution for the transmission of acoustic source signals.